UPDATE 2-FACTBOX-Experts on explosion at Japan nuclear plant

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March 12 (Reuters) - Radiation was leaking from an unstable nuclear reactor north of Tokyo on Saturday, the Japanese government said, after an explosion blew the roof off the facility following a massive earthquake.

The development has led to fears of a disastrous meltdown. Here are comments from experts about what might have happened.

PROF PADDY REGAN, PROFESSOR OF NUCLEAR PHYSICS AT THE UNIVERSITY OF SURREY

“It looks as if the coolant pumps had initially stopped working. They shut down automatically when the reactor shuts down, but there is a backup system running off a diesel generator — it looks as though that’s the bit that failed.

“As a result there is no way of pumping heat out of the reactor, so it has to cool naturally. If the reactor gets too hot, in principle this means the fuel rods can melt - but it looks unlikely this has happened to any great extent in this case.

“To reduce the pressure, you would have to release some steam into the atmosphere from the system. In that steam, there will be small but measurable amounts of radioactive nitrogen - nitrogen 16 (produced when neutrons hit water). This remains radioactive for only about 5 seconds, after which it decays to natural oxygen.

“But if any of the fuel rods have been compromised, there would be evidence of a small amount of other radioisotopes in the atmosphere called fission fragments (radio-caesium and radio-iodine).

“The amount that you measure would tell you to what degree the fuel rods have been compromised. Scientists in Japan should be able to establish this very quickly using gamma ray spectroscopy as the isotopes have characteristic decay signatures. Current reports seem consistent with a small leak to relieve pressure.”

“But we still need to establish the cause and exact location of the explosion, which is a separate issue. So far it looks like it’s not the reactor core that’s affected which would be good news.

“We must remember that there are 55 reactors in Japan and this was a huge earthquake, and as a test of the resilience and robustness of nuclear plants it seems they have withstood the effects very well.”

“By sampling the air around the station, you’d be able to tell how much radioactivity has been released. The thing they’ll be looking for more than anything is whether there’s any evidence of the fuel actually degrading,” he told Reuters.

“If the fuel is substantially intact, then there’ll be a much, much lower release of radioactivity and the explosion that’s happened might be just due to a build-up of steam in the reactor circuit.

“The most probable (cause of the explosion) is the coolant, particularly if it’s water, can overheat and turn to steam more rapidly than it was designed to cope with.”

He said it was unlikely it would develop into anything more serious, but this would depend on the integrity of the fuel, which contains nearly all the radioactivity of the plant. He said he thought it would be “pretty unlikely” that the fuel itself had been significantly damaged.

He said if this did occur, some radioactive material might be released into the primary circuit, which in turn might be vented into the containment building to release the pressure.

“Even the worse case scenario from there is the pressure in the containment building itself builds up to dangerous levels and has to be released,” he said.

“Consequently you are releasing pressure from in the containment building, some of which contains radioactivity, out into the environment. There are a lot of ifs in that chain of events.”

“The explosion at No. 1 generating set of the Fukushima nuclear plant in Japan, which took place today, will not be a repetition of the Chernobyl nuclear disaster,” Interfax quoted the Ukrainian expert as saying.

He said that the Japanese nuclear power plants use reactors of a totally different design to Chernobyl’s.

“Japan has modern-type reactors. All fission products should be isolated by the confinement (the reactor’s protection shell). Only gas emission is possible.”

Hlyhalo said that Japanese nuclear power plants are earthquake resistant.

“Apart from that, these reactors are designed to work at a high seismicity zone, although what has happened is beyond the impact the plants were designed to withstand. Therefore, the consequences should not be as serious as after the Chernobyl nuclear disaster.”

IAN HORE-LACY OF THE WORLD NUCLEAR ASSOCIATION, AN INDUSTRY BODY REPRESENTING 180 COMPANIES IN THE NUCLEAR SECTOR

“It is obviously an hydrogen explosion ... due to hydrogen igniting. If the hydrogen has ignited, then it is gone, it doesn’t pose any further threat.”

“The whole situation is quite serious but the actual hydrogen explosion doesn’t add a great deal to it.”

He said it was “most unlikely to be a major disaster” and he also did not believe there would be a full fuel meltdown.

“That would have been much more likely early yesterday in the European time. We are now 24 hours into the situatiuon and the fuel has cooled a lot in that time and the likelihood of meltdown at this stage I would think would be very, very small.”

ROBIN GRIMES, PROFESSOR OF MATERIALS PHYSICS AT IMPERIAL COLLEGE LONDON

“It does seem as if the back-up generators although they started initially to work, then failed,” Grimes, an expert in radiation damage told BBC TV.

“So it means slowly the heat and the pressure built up in this reactor. One of the things that might just have happened is a large release of that pressure. If it’s that then we’re not in such bad circumstances.

“Despite the damage to the outer structure, as long as that steel inner vessel remains intact, then the vast majority of the radiation will be contained.

“At the moment it does seem that they are still contained and it’s a release of significant steam pressure that’s caused this explosion. The key will be the monitoring of those radiation levels.”

NUCLEAR EXPERT MARK HIBBS OF THE CARNEGIE ENDOWMENT FOR INTERNATIONAL PEACE

“We don’t have any information from inside the plant. That is the problem in this case.

“If it melts down the probability that there would be a breach or that radiation would get outside of the plant because of weakness of the structure of the plant ... is much greater.” (Reporting by Michael Holden and Fredrik Dahl in Vienna)